Confused about MPPT sizing for a garden office build — am I overcomplicating this?

Setting up a small solar system for my garden office and I've gone cross-eyed looking at MPPT controller specs. I've got two 200W panels (so 400W total) wired in series, giving me a Voc of around 48V. Planning to charge a 12V lithium battery bank (two Fogstar Drift 100Ah in parallel).

I was looking at the Victron SmartSolar 100/30 but some threads I've read suggest I need to account for cold-weather Voc spikes — apparently panels can push higher voltage on a freezing British morning than the spec sheet suggests. Do I need to go up to the 100/50 just to be safe, or is the 30A output plenty for 400W into 12V?

Also wondering whether I'm better off rewiring the panels in parallel instead to keep the Voc lower and avoid any concerns about the controller input ceiling. I'd lose some of the efficiency gains from series wiring but it feels like the safer option given I'm also using this as emergency backup and really can't afford the controller frying.

Has anyone sized something similar? Would love to know what controller you landed on and whether 12V even makes sense here or if I should just go 24V from the off.

Hey @DaiLewis69, you're not overcomplicating it — Voc is just one piece of the puzzle! Don't forget to account for cold temperature derating; on a crisp winter morning your Voc could spike 10-15% higher than the datasheet figure. So your 48V could realistically hit 54-55V in practice. You'll want an MPPT with a maximum input voltage comfortably above that — most decent controllers handle 100V+ input so you've likely got plenty of headroom there. More importantly, match the controller's output current rating to your battery bank voltage. What voltage is your battery setup — 12V, 24V, or 48V? That'll determine which controller wattage rating you actually need, and whether your 400W array is properly matched. Plenty of folk overspec the controller slightly which is never a bad shout. 👍

Great thread @DaiLewis69! Building on what @Sam1965 was getting at — cold temperatures in the UK can push your Voc noticeably higher than the rated figure, sometimes 10-15% above spec on a crisp winter morning. So your 48V series string could realistically hit 54-55V at low temps.

The key thing to check is your chosen MPPT's maximum input voltage — you need comfortable headroom above that cold-weather Voc figure, not just the nominal rating.

Also worth considering your battery bank voltage when selecting the controller, as that affects which MPPT models are even compatible with your setup. What voltage is your battery system running at? 12V, 24V or 48V? That'll help narrow down whether a 40A or 60A unit makes more sense for your 400W array. You're really not far off cracking this! 👍

Great points from @Sam1965 and @TimKnight already. One thing worth adding — once you've sorted your cold-weather Voc headroom, don't forget the charge current side too. With 400W into a typical 12V or 24V battery bank, you'll want to make sure your MPPT's max charge current rating matches up. A rough rule: 400W ÷ battery voltage = max potential amps. On a 24V system that's roughly 16-17A, so a 20A controller gives you decent headroom. Going 48V panel into a 24V bank is actually a sweet spot for MPPT efficiency — the controller has plenty of voltage to work with. What voltage is your battery bank running at @DaiLewis69? That'll help narrow down whether a 20A or 30A unit makes more sense for your setup.

Great points from @Sam1965 and @TimKnight already. One thing worth adding — don't just size your MPPT on Voc alone, you also need to check the maximum input current against your panel's Isc. With 400W of panels, you'll also want to match the controller's output current to your battery voltage. For example, if you're running a 12V battery bank, you could theoretically need ~33A output, so a 40A controller gives you decent headroom. Many folk go straight for the cheapest unit without checking that figure and end up throttling their harvest. What battery voltage are you planning to run? That'll make sizing the controller much more straightforward to advise on. 👍